Does yours have the little black ball in the clear tube on the valve?
That ball is to seat when there is no longer brine coming out of the brine or salt tank.. if it is not seating then air will get into the media tank.

There is to be a ball, that is if you have the unit with that site glass or air check kit..
There is away around it, safety float assembly that has a ball in the pick up point of the float assembly.
Looks like the ball for the back wash and the brine draw are different... so they can not be traded..
You should be able to get one from herehttp://www.softenerparts.com/SearchR...Search=1030528

To the best of my knowledge there are only 2 ways that air is going to get in if there are no leaks on the valve, either from the well .. or when the unit draws brine and when the brine water has ended the plug that should happen does not.

One of the last things that your unit will do is refill the brine or salt tank with X amount of water.

When the unit goes into a reg cycle it will pull the brine out of the brine or salt tank into the softener resin, there is a ball either in the clear site glass on your valve body or the safety float assembly has a ball down at the bottom, either way when the brine is no more there is a ball to stop air from getting into the resin tank.

Failure to have a ball in either place will let the unit draw air after the brine has been drawn in.

On that type of unit you will probably see 1/3 to 1/2 the tank full of water. Do you have iron in the water and do you have a filter ahead of the softener? They're unrelated to the air issue...just a curiosity thing. Check the brine line running from the tank to the softener and make sure all connections are tight and there are no breaks in the line anywhere. It doesn't take much of a crack in that tubing to create air issues and make trouble for you.